Fluxgate type magnetometers with differential structure comprise a magnetic core, at least one active coil and a receiver coil that comprises two windings arranged in series around the core such that the magnetic field produced by an excitation current in the active coil creates opposite inductions in the windings of the receiver coil, and induced currents that cancel each other since they are also opposite. They then react to ambient magnetic fields through an induction current with a frequency equal to twice the excitation frequency provided that the active coil excitation current is sufficient. The voltage thus induced at the terminals of the receiver coil is collected and measured by the measurement means.
These magnetometers have a bias (measurement error, visible when the instrument is used in a zero field) that is variable from one instrument to another, which degrades their precision and therefore limits their use in continuous field metrology applications. Furthermore, for a given magnetometer, this bias varies for example as a function of the temperature.
One of the contributions to the bias in the case of a differential structure is related to the residual excitation frequency present at the terminals of receiver coils, because of dissymmetry between the two measurement or excitation branches. When the magnetometer is then operated in field compensation mode (conventional configuration in closed loop that in particular extends its linearity), the effect is to reinject a synchronous component of the excitation frequency with a phase shift from the excitation frequency, into the two branches of the magnetic circuit.
Dissymmetry of the magnetometer consist of irregularities of the diameter of the wire or the turns, or even the number of turns, between the windings in the receiver coil or their relative positions with respect to the excitation coils that have an influence on the mutual inductance between the excitation and induction coils.